This invention relates to a method for controlling the transmission power of a pulsed signal and a transmitter for transmitting a pulsed signal of variable transmission power. In particular, the invention relates to a location system in which a transmitter transmits such a signal by means of which it can be located by receivers of the location system.
Ultrawideband (UWB) radio is a communications technology which uses short pulses of radiofrequency (RF) energy to transfer data and perform sensing functions. By using very short pulses, UWB signals have very high bandwidth compared to transmissions from traditional radio systems, and this confers advantages in both communicating data at very high speed and performing accurate sensing functions in cluttered environments.
One use of UWB radio technology is in location sensing applications. Typically, tags are attached to the objects to be located, and a network of sensors is placed at known points in the environment. UWB signals emitted by the tags are detected by the sensors, which use them to determine measurements such as the distance from the tag to the sensor. Processing logic can then combine the known sensor positions and the measurements to determine a 2D or 3D position for the tag. Typical UWB location systems are accurate to within a few tens of centimeters, even within environments that are normally challenging for radiolocation systems, for example those with many metal reflective surfaces.
Broadband pulses used in UWB systems typically occupy a bandwidth of hundreds to thousands of Megahertz in regions of the radio spectrum below 10.6 GHz. These regions have already been allocated by regulators to other services. The output powers of UWB transmitters are consequently normally limited by regulatory restrictions to extremely low levels, so as to limit potential interference from a UWB transmitter to other users.
In order to cope with the low power level of UWB signals, UWB receivers are often coherent radio receivers designed to detect a train of broadband radio pulses sent to it by a UWB transmitter. Coherent UWB receivers are able to integrate a number of UWB pulses prior to performing signal processing and detection. Such receivers can effectively “add up” the radio energy contained in multiple pulses to increase the sensitivity of the receiver and hence the range at which a signal may be detected and decoded. In the case of UWB location systems, this allows larger volumes to be mapped out with UWB tags and sensors without increasing the power of the UWB signals.
A coherent UWB receiver is typically configured to detect the presence or absence of an incoming train of UWB pulses, and measure properties of those pulses such as their time-of-arrival and phase. One suitable form of receiver is described in U.S. Pat. No. 5,510,800 which relates to a UWB receiver for position determination applications. The receiver uses a sampling gate to mix a replica of the expected incoming pulse with the incoming signal. The mixer yields a high output response when it is triggered with a pulse replica at the exact moment when a pulse arrives at the receiver, and a low response if it is triggered when no pulse arrives at the receiver. By adding an integrating circuit on the output of the mixer, and triggering the mixer with a train of replica pulses with the same pulse repetition frequency as the expected incoming signal, it is possible to integrate multiple pulses (those which arrive at the receiver over a period determined by the time constant of the integrator) and pass the integrated signal onto subsequent circuitry configured to perform signal detection and analysis.
Some regulatory regimes further specify that UWB transmitters must implement a Transmit Power Control (TPC) function, where they reduce transmit power if they are sufficiently close to sensors that they can afford to do so without compromising system performance. Such regulations do not specify how the UWB transmit power should be reduced but conventionally control of transmission power is implemented through the use of a variable gain amplifier or variable attenuator, which can be controlled to change the output power of the signal. However, these components are expensive and occupy space within the tag, which can be problematic, particularly in the case of transmitter tags for UWB location systems. It would therefore be greatly advantageous to achieve TPC compliance without increasing the size and cost of a tag.
There is thus a need for an improved UWB transmitter for use with coherent receivers which can perform Transmit Power Control without substantially increasing the cost, size and complexity of the transmitter.